Research On In-situ Stress Measurement With Rheological Stress Recovery Method In Deep Soft Rock

In situ stress is one of the most important parameters in underground engineering. Due to the difficulty and weakness of current stress measurement methods in deep soft rock, a new one, rheological stress recovery (RSR) method to determine three-dimensional stress tensor is developed. It is supposed that rock stresses will recover gradually to be stable with time and can be measured by embedding transducers into the borehole. Then the in situ stress or the evolution of the surrounding rock stress can be determined by the stress measured by the transducer. In the RSR method, the recovery rule of the stress measured by the transducer and the measuring performance of the transducer, which will affect further development and widespread use of this method, are important problems needed to be considered further. In this paper, based on the development of in situ stress measurement methods at home and abroad, the basic theory, testing process and associated equipment of the RSR method are introduced in detail. Analytical solutions and numerical simulation are proposed to research the recovery rule of the stress measured by the transducer, and experimental investigations are carried out on the measuring performance of the transducer. Then the RSR method, as well as overcoring technique, is applied to measure the in situ stresses in Pingdingshan No.1 and No.11 coal mines in Henan Province, China. According to the actual in situ stress measurements, the reliability and related influencing factors of the RSR method are analyzed. The main research work and progress are as follows:1. Based on review of in situ stress measurement methods at home and abroad, a new one, rheological stress recovery (RSR) method to determine three-dimensional stress tensor is developed. The basic theory, testing process and associated equipment of the RSR method are systematized. The calculation method of the stress tensor and the magnitudes and directions of the principal stresses at testing point are given in detail. The design and manufacture process of the three-dimensional pressure transducer is introduced, and the working principle equation of the transducer is derived to obtain the relationship between the output signal of the transducer and the pressure on the transducer.2. The recovery rule of the stress measured by the transducer is the key of the RSR method to determine the in situ stress. Two-dimensional theoretical analysis is developed for in situ stress measurement process with RSR method in a viscoelastic surrounding rock, the rheological properties of which are depicted as both the Burger's model and 3-parameter solid model. In such conditions, explicit analytical expressions for predicting time-dependent pressures on the transducer are derived. A parametric analysis is then adopted to investigate the influences of the grout solidification time, mechanical properties of the grout layer and the rock mass. For the Burger's model, rock mechanical property has little influence on the stable recovery stress measured by the transducer, and the stable recovery stress is close to the initial stress within an error of 5% under the circumstance of appropriate grout material and transducer size. For the 3-parameter solid model, the stable recovery stress is influenced by the ratio of the long-term modulus to the initial modulus of the surrounding rock. The smaller the long-term initial modulus ratio is, the greater the stable recovery stress is, and the final recovery stress is different for different rock mass. In order to make the recovery stress restore stability as soon as possible and reach a high level, the grout material should be matching with the rock, and the solidification time of the grout should be shorten.3. A finite element model for the testing process of RSR method under different parameters of the rock mass is proposed. Rheological test data of different rocks in domestic and foreign literature are gathered to carry out the identification of Burger's model and 3-parameter solid model, and the creep parameters are obtained. Then the effects of different stress state, grout material and transducer-grout modulus ratio on the recovery stress measured by the transducer under different viscoelastic model are discussed according to the established calculation model. For the 3-parameter solid model, the stable recovery stress measured by the transducer is about 40%?70% of the initial stress for surrounding rock with grades ?, ?,and about 5% of the initial stress for surrounding rock with grades ?, ?. With the increase of the surrounding rock grade, the time of the recovery stress to be stable will decrease for ?, ?, ? grade. For the Burger's model, the stable recovery stress is close to the initial stress and almost the same when the rock grades are different. It is pointed that weak surrounding rock and good rheological properties will be beneficial to the stress recovery. Although the different stress state has an influence on the recovery stress, it can be revised in the correct range through analyze.4. The inclusion effect of the three-dimensional pressure transducer in a medium is another key of the RSR method. Numerical simulation and model test have been proposed to study the inclusion effect of the three-dimensional pressure transducer in medium. Numerical software is employed to simulate the installation process and the stress redistribution of the medium. Under different conditions of stress state and the elastic modulus ratio of three-dimensional pressure transducer to surrounding medium, the characteristics of the principal stress redistribution field and the stress distribution of each part of three-dimensional pressure transducer are analyzed. The analytical models are established with different initial stress state and modulus ratio. Through model test, this model has been verified to be correct.5. The RSR method, as well as overcoring technique, was successfully applied to measure the in situ stresses in Pingdingshan No.1 and No.11 coal mines in Henan Province, China. The device and testing process for in situ measurement are introduced briefly. The stress measurement results of RSR method and overcoring in Pingdingshan No.1 and NO.11 coal mine are given and basically in the same order. The major principal stresses are approximately in the direction of NW-SE, which correlates well with the stress regime of Pingdingshan zone known from the tectonic movement history.